1. Field of the Invention
The present invention relates to a bipolar power device having an integrated thermal protection for driving electrical loads. In particular the bipolar power device of the invention comprises a power stage including a high-voltage bipolar transistor and a bipolar or MOS type low-voltage auxiliary transistor connected in cascade. A circuit for driving the power stage is also included in the device.
By way of non-limiting example explicit reference is made in the following description to a fluorescent lamp or tube driving system comprising a pair of power devices of the type described above.
2. Discussion of the Related Art
As well known, power devices are normally used for driving electrical loads. In numerous applications and within the scope of the present invention a power device includes a three-terminal integrated component.
The present invention relates in particular to a bipolar power device. This type of device is used e.g. for high-voltage applications, even on the order of hundreds of volts and in which a MOS power device cannot be used. A preferred application of this three-terminal device is e.g. a fluorescent lamp or tube driving system.
In the power device are normally integrated monolithically a power stage and a corresponding driver circuit which controls power up and power down of the power stage depending on an applied external signal.
One shortcoming which can occur during operation of a power device is accidental temperature rise inside the device. This temperature rise can be due to a multiplicity of causes. For example, malfunctioning of the circuit driving the power stage causes excessive power dissipation and hence a further temperature rise. Sometimes damage of the cooling system, or more simply temperature increase in the surrounding due to the presence of heat sources, produces a substantial internal temperature increase. Operation of the device in these cases can be compromised. Sometimes damage is such as to make the power device unusable.
In the area of three-terminal bipolar power devices, to protect against excessive temperature rise so as to avoid the above mentioned shortcomings, apparatuses external to the device are commonly used. In a fluorescent lamp or tube driving system, a mechanical protection device, known to those skilled in the art as solder spot is used. The solder spot includes a mechanical apparatus mounted on the power device package. Usually this apparatus consists of a pair of facing metal reeds having a known thermal behavior and is placed above the heat sink connected thermally to the device. The mechanical apparatus acts as a temperature sensor and as a protection. Upon reaching an alert temperature, i.e. as soon as the temperature in the power device exceeds a predetermined maximum value, it intervenes to disconnect a power supply terminal of the device from the electrical power line of the system.
This type of thermal protection exhibits however numerous drawbacks, e.g. poor accuracy of the operating temperature and slow intervention. In addition, the protection apparatus increases the overall dimension of the power device and contributes to increasing considerably the cost of the system for the user.